Implantable medical devices (IMDs), such as cardiac pacemakers and defibrillators, are useful for management of a variety of cardiac conditions such as congestive heart failure, conduction defects and arrhythmias. Since IMDs typically deliver therapies based on sensed electrical cardiac activity, the ability of IMDs to accurately detect and interpret cardiac electrical signals is essential to the delivery of proper therapies.
Implantable medical devices typically sense cardiac electrical activity through electrodes implanted in or around the heart and/or other locations within the patient's body, which produce cardiac electrograms (EGMs). The quality of the data provided by the electrodes affects the ability of the IMD to correctly interpret the cardiac activity. The electrical cardiac signals received by the IMD may be negatively affected by factors such as pathological changes in the heart's intrinsic activity, lead maturation effects such as changes in the positioning of implanted leads, or changes in the conductive properties of the heart muscle in the region surrounding the leads, such as might result from myocardial infarction and fibrotic tissue growth around the lead. In addition, certain non-cardiac signals, such as electromagnetic noise, myopotentials, and the like, must be distinguished by the IMD from true cardiac electrical activity.
The ability of an IMD to sense cardiac signals is typically controllable by means of circuitry for adjusting the sensitivity threshold of the pacemaker's sense amplifier, such that electrical signals resulting from depolarization of the cardiac muscle must exceed this threshold in order for the cardiac event to be recognized. The sense amplifier circuitry of the IMD must be sensitive enough to ensure detection of cardiac signals, which are typically of relatively low magnitude, especially in the case of atrial sensing. However, the sense amplifier must not be so sensitive that certain non-cardiac signals, such as electromagnetic noise, myopotentials, and the like, cause the IMD to erroneously sense a cardiac signal which did not actually occur. For example, in the case of pacemakers, if the sense amplifier circuitry is not sensitive enough (undersensing), the pacemaker could lose synchronization with the natural cardiac rhythm or deliver pacing stimuli at inappropriate times. However, if the sense amplifier circuitry is set too low (oversensing), the pacemaker could erroneously sense a cardiac signal which did not occur. Similarly, defibrillators which are oversensing could detect an arrhythmia and deliver an inappropriate spurious shock. Thus, while sensitivity adjustments help to refine the ability of an IMD to detect electrical signal, undersensing, oversensing and poor signal quality create a risk that the IMD may incorrectly interpret an electrical signal. An improved system or method that provides the appropriate level of sensing is therefore desirable.